The sensory input, not the motor output, defines blink reflex conditioning

Blink reflexes following supraorbital nerve (SON) stimulation are typically modulated by conditioning stimuli (CS) to the index finger (D2) (low-intensity, prepulse inhibition paradigm) or SON (same intensity, paired-pulse paradigm). We aimed to disentangle whether CS-intensity or CS-induced motor responses define blink reflex modulation.

In 35 subjects, test SON stimuli (8 times sensory threshold, 8 × ST) were applied either alone or following CS. In experiment 1, CS were delivered to D2 with low (2 × ST) or high intensity (inducing a somatosensory blink reflex). In experiment 2, CS were applied to SON with low (<2 × ST) or test intensity. Test blink reflex size was correlated to CS-intensity and to CS-induced motor response size. Relative strength of their influence was determined in regression analyses.

Test blink reflex size showed higher inverse correlation to CS-intensity than to CS-induced motor response size for both CS delivered to D2 or SON. Regression analyses confirmed a significantly higher relative strength of CS-intensity than of CS-induced motor response size.

The sensory input of CS, rather than CS-induced motor output, defines subsequent blink reflex modulation.

This ubiquitous phenomenon calls for caution when relating the size of test responses to the size of conditioning responses particularly in paired-pulse paradigms.